The estimation of the lifetime of a turbine blade, whereby the remaining lifetime may be determined, is of great importance in the planning of maintenance intervals. Prior methods of estimation were based exclusively on operating time, as the lifetime of a turbine blade was set to an operating time during which it could, with reasonable certainty, be assumed that the turbine blade would exhibit satisfactory operation regardless of the loading exposed to the turbine blade during the operating time.
Obviously, such relatively simple lifetime estimation led to excessively frequent maintenance intervals, and the thus subsequent replacement of turbine blades that had been subjected to relative small loads during their operating time. Prior art now comprises lifetime estimation methods that to some extent are quite complicated, in which parameters such as power loading, failures in both the component being monitored and in nearby components, wear, and also faults in the measuring equipment used to measure the loading, are taken into account in addition to operating time.
For a turbine blade in a multi-stage axial compressor, it has been proven that rotating stall may cause overloading of the turbine blade with subsequent damage and compressor break-down, without the condition being detected by equipment and methods according to prior art. Rotating stall can occur in a turbine stage when the air approaches the turbine blade at the wrong angle. This may cause the flow to separate in the boundary layer between blade and air (boundary separation), whereby a varying flow is generated at one or more locations along the periphery of the stage. When a first turbine blade is subjected to this condition, the air flow is deflected towards a nearby turbine blade, which is then overloaded while the other nearby turbine blade is relieved. This causes the overloaded turbine blade to be subjected to stall, whereby the first turbine blade is relieved. Thus rotating stall propagates along the periphery of the stage at a speed of approximately half the speed of rotation of the turbine.
According to prior art the compressor is monitored by measuring its performance. The measured values resulting from the measurements form part of the input values in a lifetime estimation tool. The measurements are compared with anticipated values, as the anticipated lifetime of the component in question or the entire turbine is affected by whether the measured value is greater or smaller than an anticipated value. However, this form of monitoring is not designed to allow determination of which compressor stage is being subjected to stall.